The present invention concerns x-ray imaging, particularly methods of correcting digital x-ray images.
Since its discovery in 1885, x-ray imaging has been used to successfully diagnose the illnesses and injuries of millions of people. This form of imaging generally entails passing x-rays, a form of high-energy radiation, through a body of material onto a phosphor plate. The phosphor plate glows, or luminesces, with an intensity dependent on the material the x-rays pass through. For example, x-rays that pass through bone produce a lower intensity glow than x-rays that pass through muscle. A photographic film next to the phosphor plate chemically reacts to the glow, making a two-dimensional record of its various intensities.
In recent years, x-ray imaging systems have gone xe2x80x9cdigital,xe2x80x9d essentially replacing photographic film with electronic imaging arrays. A typical digital x-ray system includes an x-ray source, an x-ray focusing grid, and an x-ray or light detector consisting of an array of pixels. The x-ray source emits x-rays, or photons, of a specific energy level in a narrow spray pattern through a body and toward the detectors. After passing through the body, the spray pattern includes primary and scattered photons. The x-ray focusing grid, placed between the body and the detector, absorbs most scattered photons and passes most primary photons onto the array of detector pixels.
In response, each detector pixel in the array provides an electrical output signal representative of the intensity of light or x-rays striking it. Each output signal is then converted to a number known as a digital pixel value, which is in turn output as a particular color on an electronic display or printing device, enabling viewing of the x-ray image.
Before display, it is common to correct the x-ray image for irregularities in the array of detectors. These irregularities, stemming from the uniqueness of each detector pixel in the array, lead the detector to output different signals in response to the same incident light or x-rays. Correcting the image typically entails adjusting the digital representation of each detector output signal by an experimentally determined number for that detector. The numbers for all the detector pixels, known collectively as a correction map, are usually stored in a digital memory of the x-ray system.
In addition to correcting for detector irregularities, the correction map also corrects for all other system sensitivity factors, such as non-uniform x-ray field and grid artifacts, affecting formation of a particular x-ray image. Because of the complex interdependency of the many factors affecting system sensitivity, every correction map is uniquely applicable to a specific system configuration and exposure technique, that is, to a specific set of system factors. Moreover, configuration and technique changesxe2x80x94such as increasing or decreasing x-ray tube voltage (kVp) and x-ray beam filter, or replacing one grid with anotherxe2x80x94that are made to tailor the system to specific imaging applications require use of different correction maps. Skulls, chests, and hands, for example, generally require different exposure techniques and grid types and thus different correction maps for best results.
Therefore, to support a wide variety of system configurations, digital x-ray imaging systems may store and use many application-specific correction maps. For example, if a system supports N different configurations and exposure techniques and P different grid options, it may store Nxc3x97P (N times P) different correction maps to correct images made under all possible grid-and-technique combinations.
One problem with storing many application-specific correction maps is that they all require repeated maintenance or update to adjust for wear, age, and other time-varying characteristics of components in host x-ray systems. Updating, or recalibration, of many correction maps is not only time-consuming but expensive in terms of system downtime. Moreover, new x-ray applications and grid types are continually being developed, further expanding the number of correction maps requiring storage and update. Accordingly, there is a need for better correction methods and systems.
To address this and other needs, the inventors devised new methods and apparatus for correcting images in digital x-ray imaging systems. In systems which, for example, support N different non-grid configurations and P different grids and thus would conventionally require storage and update of Nxc3x97P (N times P) different correction maps, these exemplary methods and apparatus in accord with the invention facilitate the same correction capability with storage of only N+P (N plus P) different correction maps. With storage of fewer correction map, systems incorporating various embodiments of the invention, ultimately require considerably less time and expense for recalibration.
One exemplary method determines grid-only and non-grid correction maps and corrects images based on a combinations of these xe2x80x9cpartialxe2x80x9d or xe2x80x9cmodularxe2x80x9d correction maps. More particularly, this exemplary method determines a grid-only correction map from first and second flat-field images, the first made without a grid and the second with a grid. The first image is used to determine the non-grid correction map, and both images are used to determine the grid-only correction map.
The exemplary apparatus includes a memory which stores one or more non-grid correction maps and one or more grid-only correction maps. Also included is software for selecting one or more of the non-grid correction maps and one of the grid-only correction maps and for correcting a given image using the selected non-grid and grid-only correction maps.